1. Technical Field
The present invention relates to acoustic sensors and methods for manufacturing the same, and specifically to a MEMS (Micro Electro Mechanical Systems) type acoustic sensor manufactured using a MEMS technique, and a method for manufacturing the same.
2. Related Art
The MEMS acoustic sensor is disclosed in Japanese Unexamined Patent Publication Nos. 2008-301434 and 2009-89097, for example.
(Regarding Japanese Unexamined Patent Publication No. 2008-301434)
FIGS. 1A and 1B are schematic cross-sectional views showing a state in which an acoustic sensor disclosed in FIG. 7 and FIG. 8 of Japanese Unexamined Patent Publication No. 2008-301434 is mounted in a package. The acoustic sensor 11 has a diaphragm 14 (vibration thin film) laminated on a silicon substrate 12 with a back chamber 13 opened, and a back plate 15 arranged on the silicon substrate 12 to cover the diaphragm 14. Such an acoustic sensor 11 is normally mounted in a package 17 along with an IC circuit 16, as shown in FIGS. 1A and 1B, where an electrode pad 18 of the acoustic sensor 11 and the IC circuit 16 are wire connected with a bonding wire 19, and the IC circuit 16 is wire connected to an electrode section 21 of the package 17 with a bonding wire 20.
In such an acoustic sensor 11, however, the outer peripheral part at the upper surface of the silicon substrate 12 is completely exposed. Therefore, as shown in FIG. 1A, if the bonding wire 19 connecting the acoustic sensor 11 and the IC circuit 16 bends downward due to some kind of load, the bonding wire 19 may come into contact with the silicon substrate 12, thereby causing short circuit in the acoustic sensor 11. Even if the bonding wire 19 is not bent, if a foreign substance 22 (e.g., microscopic dust etc.) gets caught between the bonding wire 19 and the upper surface of the silicon substrate 12 as shown in FIG. 1B, short circuit may occur between the electrode pad 18 and the silicon substrate 12 through the foreign substance 22.
Because the IC circuit 16 is entirely sealed with a resin 23, the bonding wire 19 is fixed by the resin 23 so that the bonding wire 19 does not bend and the foreign substance does not enter, but the acoustic sensor 11 which is the sound receiving unit cannot be sealed with resin as acoustic vibration will be shielded if sealed with resin.
In the acoustic sensor disclosed in FIG. 4 and FIG. 5 of Japanese Unexamined Patent Publication No. 2008-301434, the surface of the silicon substrate is covered with an insulation coating (SiO2 film), but the reliability is low for preventing short circuit because the insulation coating is thin. In the cross-sectional view shown in FIG. 6 of Japanese Unexamined Patent Publication No. 2008-301434, the back plate is extended up to the edge of the silicon substrate. However, this is a view in which the outer peripheral part of the silicon substrate where the upper surface is exposed is omitted, as apparent from the cross-sectional view of FIG. 4 of Japanese Unexamined Patent Publication No. 2008-301434, and the back plate is not actually extended up to the edge of the silicon substrate.
(Regarding Japanese Unexamined Patent Publication No. 2009-89097)
FIG. 2 of Japanese Unexamined Patent Publication No. 2009-89097 discloses an acoustic sensor in which a structural object is arranged up to the outer peripheral edge of the upper surface of the silicon substrate to cover the upper surface of the silicon substrate. In such an acoustic sensor, however, the outer peripheral part of the upper surface of the silicon substrate is covered with a member (surface layer protective film etc.) different from the back plate (plate including upper layer conductive film). The man hours required during manufacturing thus increases, and the productivity is not satisfactory.